Frequency shift radio telegraph system



May 30, 1950 COOK ETAL 2,509,212

' FREQUENCY SHIFT RADIO TELEGRAPH SYSTEM Filed March 1, 1 947 F/G. RECEIVERS BANDPASSFILTER LIMITER FILTER LOCAL OSCILLATOR 6 OUTPUT l3 TIME CONSTANTCircuH DlSCRIMINATOR .J. CONTROL UTILIZATION DEVICE 8 5 DEVICE RECTIFIER FILTER (FILTER \RECEIVERS UMIIER, FlLTER UTILIZATIONDEVICE I I kggflLLATOR P I BAND PASS FILTER flZA f 6 OUTPUT l7 /4 4 J AMPLIFIER (FILTER FILTER /5 /7a FREQUENCY'CHANGER/ a r I f 4 166 TIME CONSTANT LOCAL ,IAMPLIFIER CIRCUIT OSCILLATOR V CONTROL DEVICE 9 RECTIFIER DISCRIMINAT OR Patented May 30, 1950 FREQUENCY SHIFT RADIO TELEGRAPH SYSTEM Arthur Cook, Clapham, London, and Herbert Dunn Bickley, Welwyn Garden City, England, assignors to International Standard Electric Corporation, New York, N. Y.

Application March 1, 1947, Serial No. 731,826 In Great Britain February 5, 1946 Section 1, Public Law 690, August 8, 1946 Patent expires February 5, 1966 4 Claims.

This invention relates to carrier-wave telegraph systems of the kind in which signals are sent by changing the frequency of the emitted wave.

In such systems unwanted frequency changes may occur either in the transmitter or the receiver and it is usual to provide at the receiver arrangements for compensating for such unwanted frequency variations, by correcting the tuning of the receiver. It is, however, necessary to arrange that the correcting device is not affected by the changes of frequency due to signalling but only by unwanted changes, and the usual arrangement is to allow only one of the frequencies used in signalling to control the correcting device through a circuit so arranged that the control is quickly established when this frequency is received but dies away comparatively slowly at the cessation of the receipt of said frequency.

According to one feature of the present invention we provide a receiver for a carrier wave telegraph system of the kind specified in which means is provided for correcting, for unwanted changes in freqency during the reception of any of the frequencies used in the system.

According to another feature of the present invention we provide a receiver for a carrier wave telegraph system of the kind specified in which means is provided for controlling the tuning of the receiver by means of a portion of the energy received during the reception of one of the frequencies used in the system and means is also provided for altering the frequency of a portion of the energy received during the reception of another of the frequencies received and for using also the energy of said altered frequency to control the tuning of the receiver.

The invention will be more fully understood from the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic diagram of a known receiver of the kind to which this invention relates.

Fig. 2 is a schematic diagram of a receiver similar to that of Fig. 1 but arranged in accordance with the present invention.

Referring to the drawings, the receiver shown in Fig. 1 is a receiver for a radio telegraph system of the kind in which Morse code signalling takes place by the use of frequencies f and fiAf. The frequency f is emitted from the transmitter during idle periods, and is called the spacing frequency and during signalling the frequency is changed to fiAf for the duration of the marking elements of the code, returning to the value f for the duration of each spacing element of the code.

The signals are passed from the antenna to a receiver l of the superheterodyne type l3 being the local oscillator associated with a frequency changer in the receiver l. The signals at the changed frequency are passed through a band pass filter 2 in order to eliminate unwanted frequencies and thus improve the signal to noise ratio, and then through a limiter 3 the purpose of which is to remove variations in amplitude of the signals. After passing through the limiter 3 the signals are passed to filters 4 and 5 in parallel which select the frequencies f and f'iAf which are those frequencies to which the spacing and marking frequencies f and fi-Af respectively are changed by the frequency changer stage I. The frequencies f and fztAf are then passed to the device 6 wherein the signals are converted into D. C. signals passed to an output 1. From the output of limiter 3, a small band of frequencies on either side of frequency J" is selected by a filter 8, which, however, rejects the frequency ,fiAf. The output of filter 8 is passed through a discriminator 9 and rectified by a rectifier Ill. At the output of rectifier II] a steady potential difference appears of amplitude dependent upon the precise value of the frequency f of the spacing signal. This D. 0. potential is applied through a time constant circuit l I to a control device l2 whereby the frequency of the local oscillator l3'1s made to vary in the proper sense to correct for the variation in the frequency at the output of filter 8. The time constant of circuit II is of such value that any steady potential difference applied to the control device 12 will persist for the duration of a marking element provided that this is not of too long duration.

The above described arrangement suffers from various disadvantages. The magnitude of the time-constant of the circuit ll limits the maximum rate of change of frequency which can be dealt with by the correcting device. change of frequency occurring during a marking period goes uncorrected. This is particularly disadvantageous if it be required to send long marking signals, e. g. of several seconds duration as may be required to operate remote control devices in certain tape facsimile systems of telegraphy. If the time constant of circuit H is made sufllciently long to hold the frequency of the local oscillator l3 at the same value throughout such a long marking period, then the rate of frequency correction becomes very low, and if this time constant is not long enough for this purpose, then the circuits remain entirely uncorrected for some period of time. In either event the correcting device may lose control and the; receiver may adjust itself toan unwanted signal on a neighbouring frequency channel.

Conditions in which long marking periods occur are also met if it be required-to signalby hand-speed Morse, and this is sometimes required even over telegraph circuits normally used for higher speed working by Morse, teleprinter or other code.

The above mentioned difficulties are overcome by the arrangement shown in Fig. 2 which shows one embodiment of the invention'as applied -to a receiver similar to that of Fig. l. superheterodyne receiver l is used, with a local oscillator I 3 connected to a band pass filter 2 and limiter 3. The filter 4 selects the frequency f in the same way as in Fig. 1 but instead of part of the output of limiter -3 going to filters 5 and 8,

itgoes. to filter l land a frequency changer. A local oscillator I5 is associated with the frequency-changer- 14, the frequency thereof being made; equal to A Thefrequency changer M is connected to filters 4a.-and Ila. Filter-4a has aband pass characteristic thesame as that of filter time. both filters-Land laselect the frequencyj'.

Whenthe spacing frequency f is received, the frequency .at the input of frequency changer M will be f'qand the frequencies, at the output thereof will :be -f'+:Af and f- -Af. Neither of these is passedby filter 4a andxthus the deviceifi is affected only on the spacing side i. e. at the output of filter-4.

Filters I7 and He have pass characteristics similar to that of filter 8 ofFig. l i. e. each passes a band of frequencies centered on frequency I, but =Will-not-pass either f+Af or f'Af. During the receipt of spacing frequency f, filter ll. passes a portion ofthe outputfrom limiter3, :andthis, after passin through an amplifier 16 or other suitable one-way device, is-used as in Fig. l to operatethe control device [2. No energy of frequency 1- however, reaches filter l-la.

-When the marking frequencyfiAf is received, the-output of frequency changer l is f'i-Af and no energy passes the filterA or the filter IT. The frequencies at :the output of frequency changer M are f+Af+Af-andf'+Af-A,f ipe. f-+-2Af and f inthe one case (if-themarking frequency is f+-A}) or f-A +Af and f' AfAf i. e. and ,f 2Aj in the other case (if the anarking frequency'is i-A The frequency ,fpasses filter 4a and is passed to the ,dev-icefi as the marking frequency. The frequency falsopasses filter Har and after passing through-an amplifier or other suitable one-way-dev-ice Hid is applied to thecontrol device l2 in the same way as isthe output of filter i7 during spacing periods.

The correction for unwanted frequency changes thus takes place both during spacing and marking periods "and the time-constant of the circuit -H can beimadeamuchshorter than in the case of Fig. l. Thepurpose of the one-way devices 16 and 16a is to enable the outputs. of filters I1 and Ma to be both connected tothe input of the limiterii.

It should be noted that it isessential that, if

an unwanted change of frequency 6 should occur the 1 corresponding change in frequency at the As before a [5 to be 2f'+Af but in this case an unwanted frequency "+A;f+5f is changed to f5f.

The band-width of filters I1 and I'm. can with advantage be greater than that of filters 4 and 4a. In many cases it may be desirable to keep the band-width of these latter filters as narrow as is practicable in order to exclude noise interference, but the band-width of filters I1 and Na is advantageously made wider to allow of automatic correction of wide changes of frequency in the signals. The bandwidth of each of the filters l'lrand Ila-may extend from f'- AJ to f'+. ,Af.

It should also be noted that the circuit of Fig.2 hasqa further advantage over that of Fig. Lin that fewer changes are required if it be desired to receive a transmission in which the frequency change between spacing and marking is some other value than A), say (pf. In Fig. 2 it is only necessary for this purpose to change the frequency of the local oscillator [5 to f, the filters 4 and 4a. and, in most cases, fiiters H and lla remaining unchanged.

It will be clear that the invention is applicable to systems in which the signals are constituted by three or more conditions.

What is claimed is:

1. A receiver for frequency shift markingand spacing carrier waves comprising a frequency changer, said frequency changer comprising a first local oscillator for mixing with received waves for providing intermediate frequency waves, a plurality of electrical paths connectedin parallel to the output of said frequency changer, a utilization device, a first one of said paths com-. prising a first band pass filter for passingintermediate frequency waves of one frequency tosaid utilization device, a second band pass filter for passing said intermediate frequency waves of one frequency, .a second one of said paths comprising afrequency changer having. a secondlocaloscillator generating waves of a frequency equal to the difference between the marking and spacing frequencies, means formixing said intermediate frequency waves with saidlocal oscillator waves, a third and a fourth band pass filter in parallel coupled to the output of said mixing means,.a means connecting the output of the third band pass filter to said utilizationdevice, means for applying the outputs of the second and the fourth band pass filters to a common electrical path for correcting the tuning of said first local oscillator, the center pass frequency of all of said band pass filters being the same.

2. Receiver as claimed in claim 1 in which the pass bands of said first and third band pass filters are identical. Y

3. Receiver as claimed in claim 1 in which the pass bands of said second and fourth band pass filters are identical and are Wider than the pass band of said first and third band pass filters.

4. A receiver for frequency shift carrier waves of two different signal frequencies comprising a first local 0scil1ator,-,means for mixing waves from saidlocal oscillator with received carrier waves to provide intermediate frequency waves, four filter circuits for passing intermediate frequency waves corresponding to one signal frequency, a utilization device, a second local oscillator tuned to the difference circuit frequency between said signals, a tuning control circuit for said first named local oscillator, means for mixing said intermediate frequency waves with said second local oscillator, means for applying said mixed waves through one of said filters to said utilization device and through a second filter to said tuning control circuit, means for applying said intermediate frequency waves through a third filter to said utilization device and through a fourth filter to said tuning control circuit.

ARTHUR COOK. HERBERT DUNN BICKLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Round Sept. 13, 1927' Peterson Feb. 22, 1938 Humby et a1 Aug. 20, 1940 Katzin Feb. 10, 1941 Whitaker Apr. 26, 1941 Peterson Apr. 6, 1943 Peterson Feb. 15, 1944 Earp Sept. 19, 1944 Davey Sept. 11, 1945 Hysko June 4, 1946 Earp Dec. 30, 1947 

